Whenever sulfur-, nitrogen- and chlorine-containing fuels, for example, are burned during combustion processes, sulfur oxides, nitrogen oxides and hydrochloric acid, as well as chlorinated organic compounds, such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), are released. Due to their toxicity, all these pollutants are often subject to national emission limits. In the Federal Republic of Germany, for example, emission limits have been laid down by the legislator in the 17th Ordinance Implementing the Federal Immission Control Act (17th BlmSchV) for waste incineration plants.
In industrial combustion systems, solid fuels are usually burned in two stages. In a first step, the solid fuel is burned by adding an oxygen-containing primary gas (primary air). The oxygen-containing primary gas (primary air) is typically supplied in substoichiometric amounts. Because of the resulting incomplete burnout of the primarily formed flue gases, which is attributable to a local lack of oxygen in the combustion bed, oxygen-containing secondary gas (secondary air) has to be introduced into and mixed with the primarily formed hot flue gas, which still has a high heating value, in a superstoichiometric ratio, as a result of which a post-combustion process is initiated.
Solid fuels, such as household waste and biomass, but also coal, are often burned in grate-, fluidized-bed, or also rotary kiln combustion systems. Household waste and biomass, in particular, are often highly inhomogeneous in terms of heating value, ash content, moisture content, material composition and/or particle size.
In grate combustion systems, the solid fuel is transported on a combustion grate subdivided into several zones, and is burned out while primary air is supplied to the individual grate zones in a controlled manner. In most cases, this done without subjecting the solid fuel to complex preprocessing. In an idealized consideration, the burning of solid fuels can be broken down into the sequential subprocesses of drying, degassing and combustion of the fixed carbon. Due to the typically poor mixing of the often inhomogeneous solid fuels or fuel mixtures in the fuel bed, these subprocesses may overlap during transport on combustion grates. Drying occurs mainly in the front grate area, and is caused by intensive heat radiation from the hot combustion gases from the primary combustion chamber and/or from the hot combustion chamber walls/ceiling and/or by supplying preheated primary air. During drying, the oxygen supplied with the primary air is not consumed. Further temperature increase during the subsequent degassing of the fuel causes large amounts of volatile hydrocarbons to be released from the fuel bed. This is the grate area where the highest local carbon conversion occurs. Depending on the local temperatures and the O2 concentration in the fuel bed, the released hydrocarbons are ignited and completely or partially burned. When oxygen is supplied in (locally) substoichiometric amounts, considerable amounts of unburned hydrocarbons remain in the exhaust gas after the oxygen present in the primary combustion zone is fully consumed. These unburned hydrocarbons are partially converted to CO, H2 and soot by gasification reactions at high temperatures. These primarily formed flue gases in the primary combustion zone have a high heating value.
In parallel to the degassing process, volatile nitrogen-containing compounds (N species), mainly NH3 (ammonia) and, to a lesser extent, HCN (hydrocyanic acid) and nitrogen-containing hydrocarbons are formed from the fuel nitrogen. These primary N species are completely or partially oxidized to NO, depending on the local O2 concentrations and temperatures in the combustion bed. When there is a lack of oxygen (in the main combustion zone), considerable amounts of volatile nitrogen compounds, in particular NH3, remain in the high heating-value flue gas exiting the combustion bed.
The thermally unstable sulfur-containing compounds of the fuel are mainly released as hydrogen sulfide (H2S) when there is a lack of oxygen in the area of the primary combustion zone.
During combustion of the solids, the chlorine-containing compounds of the fuel (e.g., PVC and from inorganic chlorides, such as NaCl) are mainly converted to hydrochloric acid. A smaller fraction is released into the flue gas as volatile inorganic chlorides (e.g., alkali chlorides, heavy metal chlorides) or also organic chlorine compounds (e.g., chlorobenzenes).
In the rear grate area, the fixed carbon remaining after the degassing process is burned while primary air is supplied in locally superstoichiometric amounts. Since the primary air is typically supplied in considerable excess in this area of the grate, the combustion bed temperature decreases, as a result of which the kinetics of the conversion of residual carbon is relatively slow. Rising temperatures in the slag bed at the end of the grate accelerate the combustion of carbon when oxygen is present in sufficient amounts, thus ensuring low residual carbon contents (TOC) in the discharged slag.
The nitrogen content of the residual coke formed upon degassing is relatively low. Combustion with an excess of O2 mainly produces NO (nitric oxide). The volume and distribution of primary air, and the grate kinematics, have a significant influence on the progress of combustion of the fuel bed as it transported along the grate, thus influencing the axial distribution of temperatures, O2 concentrations, flue gas heating values, and the NH3/NO ratio in the flue gas streams released from the individual combustion bed zones.
The flue gases formed primarily during combustion of the solids, in particular the oxygen-free, high heating-value flue gases from the primary combustion zone (O2 minimum) must be burned as completely as possible at high temperatures in a second combustion step by addition of and mixing with superstoichiometric amounts of oxygen-containing secondary gas (secondary air).
In the area of this flue gas burnout zone, complex reactions ultimately result in nitrogen oxides (NOx, mainly nitric oxide NO) and/or nitrous oxide (N2O) and/or nitrogen (N2) being formed from the N species primarily formed during combustion of the solids. The heating value and the NH3/NO ratio of the flue gases before entering the primary flue gas burnout zone, and the local distribution of temperatures and oxygen concentrations during flue gas burnout, exert a decisive influence on the resulting final distribution of N species in the flue gas downstream of the flue gas burnout zone. Under ideal conditions, NH3 and NO react to produce N2 during flue gas burnout as a result of an autogenous SNCR process.
In grate combustion systems for household waste incineration, the stoichiometry of the primary air supplied (the sum of all primary air streams) is typically in the range from 0.6 to 1.2. In waste incineration plants, the secondary air is controlled in such a way that the combustion temperatures in the flue gas after addition of secondary gas is maintained above 850° C. for a residence time of two seconds. The oxygen content in the spent flue gas is typically in the range from about 5 to 12 percent by volume. The energy released during combustion is typically used to generate steam in a boiler. The often relatively high excess of air and the typically relatively high temperatures in the flue gas downstream of the boiler (180-250° C.; i.e., above the acid dew point) cause a considerable loss of energy during the recovery of the thermal energy contained in the flue gas in a boiler. The boiler efficiency (the ratio of the energy content of the generated steam to the energy input of the fuel) is in the range of 80-85% in waste combustion and about 93% in coal combustion.
There are various well-known approaches for reducing pollutant emissions in combustion processes. These approaches not only include downstream flue gas cleaning measures, but also primary measures for reducing the pollutant formation rates.
German Patent DE 103 38 752 B9, for example, describes a method for reducing polyhalogenated compounds, such as PCDD/Fs, in incineration plants having at least one combustion chamber. In this approach, SO2 is selectively separated from the flue gas in at least one scrubber and is recycled into the combustion chamber. The sulfation of the chloride-containing fly ash caused by increasing the SO2 concentration (in the flue gas downstream of the flue gas burnout zone) significantly reduces PCDD/F formation. In addition, sulfated fly ash having a low chloride concentration cause considerably less corrosion problems for the boiler materials.
Further, German Patent DE 10 2006 016 963 B3 describes a method, in which sulfur dioxide SO2 is selectively separated from the flue gas in at least one scrubber by means of ammonia or ammonia compounds, thereby forming an aqueous ammonium sulfate/sulfite solution, which is recycled entirely or partly into the combustion chamber, and which during thermal decomposition also increases the SO2 concentration.
Furthermore, German Patent DE 10 2006 005 464 B3 describes a method for reducing NOx on the primary side thorough axial mixing of all flue gas streams emanating from the fuel bed in a grate combustion system prior to entry into the flue gas burnout zone, and simultaneous control of the temperature by means of a controlled gas/water free jet.
Moreover, various approaches have been proposed in the literature to increase energy efficiency by combining several different combustion processes.
German Patent Application DE 10 2005 036 792 A1, for example, describes a system wherein combustion takes place in two separate plants, but the flue gases are at least in part cleaned together. In this approach, a first combustion plant (for waste, biomass or other substitute fuels) and a second combustion plant, which is fired with fossil fuels (such as hard coal, lignite, natural gas, oil), are coupled together on the steam side. Chlorine-containing fuels, in particular, produce highly corrosive flue gases during combustion. In order to limit corrosion in the boiler, saturated or slightly superheated steam at relatively low temperature levels is generated in the waste combustion plant. The relatively energy primary steam is then further superheated in the second, fossil-fired combustion plant so as to increase the efficiency in steam-powered generation of electricity. However, the disclosed concept is very complex and requires two different fuel streams to be simultaneously supplied to two separate combustion plants.
German Patent DE 43 00 192 C2 proposes to combine two waste heat processes for generating superheated high energy steam. The first process may be a waste combustion process and is used to generate saturated steam. In the second process, the saturated steam is superheated in a boiler, which may be heated by the exhaust of a gas turbine, for example. This concept also requires two different fuel streams.
European Patent EP 0 593 999 B1 and German Patent DE 19 15 852 C3 also describe methods for power generation in waste or hazardous waste incineration plants. Saturated steam is generated by the combustion of waste and is superheated in a second boiler using regular fuel, such as natural gas (EP 0 593 999 B1) or oil or coal (DE 19 15 852 C3); i.e., also by means of a second, fossil fuel.
In contrast, European Patent EP 0 823 590 B1 describes a method for generating steam having a temperature of 200-320° C., which method uses a chlorine-containing energy source (e.g., waste) and, unlike the aforementioned prior art, uses only one fuel. Shredded waste is heated in a fluidized-bed pyrolysis plant while air is introduced at a low rate. In a first combustion process, the chlorine-containing pyrolysis gases released during the pyrolysis of the waste are burned, and steam having a temperature below 400° C. is generated. During combustion, the nitrogen compounds contained in the pyrolysis gas form significant amounts of nitrogen oxides and must be separated using complex flue gas cleaning techniques. The coke-containing pyrolysis residue is subsequently mechanically processed (screened to remove coarse fractions), and after removing impurities, it should be substantially free of chlorine. In a second stage, this residual coke (fixed carbon) from the pyrolysis of waste is burned, thereby further superheating the steam to up to 520° C. However, pyrolysis cokes from waste materials typically still contain considerable amounts of alkali and/or metal compounds, in particular chlorides, which are released into the exhaust gas during combustion, and which may deposit on the heat-exchange surfaces of the steam superheater. This causes the well-known corrosion effects in the boiler.